Recently, vehicles have been produced with brake systems that can in certain situations actively apply brakes to individual wheels in an effort to improve vehicle stability or handling near and at the limit of adhesion. These systems usually seek to bring the vehicle as closely as possible to a desired path and/or to minimize the lateral movement of the tires relative to the road surface. Typically, the control is configured to bring the vehicle yaw and/or lateral velocity (or side slip angle) into correspondence with desired yaw and/or lateral velocity values.
Because the price of yaw rate sensors is still relatively high, active controls of the type described above are necessarily limited to a small number of vehicles. While the vehicle yaw rate can be estimated, either based on measured speeds of the un-driven wheels or measured lateral acceleration, the estimates often fail to faithfully track the actual vehicle yaw, particularly during braking or when the vehicle deviates from a steady state condition. What is desired is an active brake control that does not require a yaw rate sensor, but that can reliably estimate and control yaw rate and side slip angle even during conditions that degrade the validity of an estimated or computed yaw rate value.